This invention relates to nonwoven fabrics which may be produced in compact or low bulk roll form and later bulked by the end user by application of heat.
Nonwovens are widely used in disposable garments such as diapers or others sanitary goods having a liquid absorbing element. Such items necessarily include a top or cover sheet which resides between the skin surface and the inner absorbent layer or layers. The function of the cover sheet is to provide an outwardly facing surface having maximum softness. Also, this sheet must be porous and have maximum bulk or thickness per unit weight to permit rapid draining away of liquids from the body into the liquid absorbing core and providing a liquid insulating effect and dry feel.
In order to increase the bulk or thickness of nonwoven fabrics, it is known to include quantities of heat shrinkable fibers, such as bicomponent fibers, which shrink, curl or otherwise contract when heated to a certain temperature. Many varieties, such as sheath/core and side-by-side bicomponent types, as well as single component types, are well known. In some cases, the bicomponent fibers are pre-crimped to achieve bulk, and a component of the fiber is melted to attain bonding. In other cases, the bicomponent fiber is heat activated with other fibers to increase bulk.
EP 181300 describes a method in which low shrinkage fibers are uniformly blended with high shrinkage fibers and formed into a fabric. The fabric is then heated at spaced locations to cause bulking of the fabric in a pattern. To achieve the desired bulk, 60-75% of the fibers are of the bicomponent type.
British patent 1334735 discloses the concept of securing a plurality of heat shrinkable threads by an adhesive to a fabric, and subsequently heating the composite.
Most so-called high luft cover fabrics are bulked during the process of manufacture. As a practical matter, these bulked fabrics must be wound into rolls, slit into narrow widths, and shipped to a converter for conversion into the component for a sanitary article. If the web is initially formed in a bulked state, this unnecessarily increases transportation costs, as well as handling costs, since the amount of material available on one roll is limited, necessitating frequent roll changes by the manufacturer and the converter.
In view of the above, a recent important goal in this field of nonwovens is to provide a fabric which can be manufactured and shipped with the minimal thickness and then later be bulked on line by the converter, preferably by heating or some other expedient means.
One proposed solution to this objective is to subject the bulked web, as produced, to compression or tension prior to final winding on rolls prior to shipment. This requires that the web must have capability of being restored to at least a portion of its original bulk.
An example of compression technique is described in U.S. Pat. No. 4,601,937 wherein the web is heated and compacted in a fashion such that it can be rebulked by steaming. U.S. Pat. No. 5,143,779 discloses a fabric which is produced in an initial lofty state and subjected to compression or tension while winding onto a roll. The fabric is later subjected to heat to rebulk the fabric. Both of these described processes apparently rely on the memory of fibers in its original bulked form, and this may vary over time and handling conditions.